This invention relates in general to a method for charging a furnace, and more particularly to a method for the self-activated recharging of a furnace melt.
Many types of crystalline ingots are pulled from a melt by a process such as the Czochralski method. This is the case, for example, for most of the silicon crystals used in the semiconductor industry. Typically, in the Czochralski method, a crucible is charged with raw material such as pure polycrystalline silicon. The charge is melted in a furnace under carefully controlled atmospheric conditions which can include inert ambients to prevent oxidation. A crystal is pulled from the melt by well-known techniques which include dipping an oriented seed crystal into the melt and controllably withdrawing the seed to nucleate growth of a high quality single crystalline ingot. After growth of an ingot some melt usually remains in the crucible. To grow an additional ingot the furnace and crucible must be cooled to room temperature, the old melt removed, and the crucible recharged. This must be done at room temperature or at least at a reduced temperature because the melt cannot be exposed to an oxidizing and contaminating room ambient while at a high temperature. Cycling between high and low temperatures often results in destruction of the crucible, especially when the severe temperature cycling is accompanied by a large mismatch in thermal coefficient of expansion between the crucible and the remaining melt. The temperature cycling inevitably results in a costly loss of time and energy.
To prevent the destruction of crucibles, to more fully utilize furnace equipment, and to be more energy conservative, it is therefore desirable to have an ingot-pulling process which does not require cooling between cycles. An apparatus for carrying out such a process was disclosed in U.S. Pat. No. 2,975,036. The apparatus includes a double-chamber structure, with one chamber for growing the ingot, the other for facilitating ambient control during loading and unloading. The ambient can be separately controlled in each chamber; the two chambers are separated by a movable isolation gate valve.
One problem with such an apparatus, however, relates to recharging the melt. Recharge material can be easily loaded into the loading chamber of the apparatus, the chamber can be purged and filled with the desired ambient, but the charge material cannot simply be dropped into the crucible as this would cause severe problems with splashing of extremely high temperature material. The aforementioned U.S. Pat. No. 2,975,036 suggests the use of a mechanical clamping apparatus such as tongs, but such mechanical means are susceptible to sticking, locking, or other failure at the high temperatures encountered. The clamping apparatus also has no means for introducing a controlled amount of dopant with the charge material. Alternate recharging methods are illustrated in U.S. Pat. Nos. 3,031,275 and 4,036,595. These methods, however, are not without their problems and limitations including, for example, difficulties associated with handling large pieces of charge material, controlling ambient, conveying high temperature molten material, and adding a precise amount of dopant.
Accordingly, in view of the deficiencies of prior art methods, it is an object of this invention to provide a self-release method for charging or recharging a furnace.
It is a further object of this invention to provide a reusable apparatus capable of reliably recharging a melt with large pieces of charge material without excessive melt splashing.
It is a further object of this invention to provide an improved method for charging a furnace without first cooling that furnace.
It is a further object of this invention to provide a method for recharging a furnace with a precise doping concentration.